Heretofore, thin film magnetic heads have been widely employed in various devices such as magnetic disk drives or the like. The thin film magnetic head is conventionally produced by using various techniques such as vacuum film forming, plating, photolithography, etching, etc. to form magnetic circuits, coils and other elements, and then forming a protective film thereon by sputtering a metal oxide thin film such as alumina and silica.
In the case where the thin film magnetic head is used in a large scale device such as a hard disk drive in a flying type or other non-contact magnetic head, it is usually housed in a sealed space and floated so that there is no need of using other protective measures than the above mentioned protective film if there is no contact of the head with other objects.
However, with the development of thin film magnetic head techniques, the thin film magnetic head finds various applications in home electric appliances. However, in such applications as a video tape recorder or a magnetic disk device in which a flexible magnetic recording medium is utilized, the magnetic head is not sealed any more and is exposed in the device unlike the hard disk drive, and is usually used in contact with a magnetic disk or tape. Accordingly, the conventional means for protecting the thin film magnetic head, avoiding its wear and mantaining an ideal contact between the head and the recording medium, was to use a further protective plate which is bonded to the surface of the thin film magnetic head to form a sandwich structure, having a generally spherical smooth surface.
In order to bond a thick protective plate on to the thin film magnetic head, the glass bonding process is often used in light of the durability and the bonding strength of glass. According to this method, a low melting point glass having a low softening temperature in the form of a thin rod or sheet is placed between the thin film magnetic head and the protective plate, the assembly is pressed together and heat-bonded. The fusing temperature during this operation is usually about 380.degree. C. at the lowest and normally about 400.degree. C. Therefore, it is essential that the thin film magnetic head can withstand this high temperature. However, known materials which are not impared in their properties under such high temperature condition are only those of sendust (for example, Fe85-A15-Si10) and permalloy (for example, Ni80-Fe20).
Superior in magnetic properties to these magnetic materials are amorphous alloys (for example, Co85-Nb10-Zr5) which, however, start partial crystallization when the protective plate is subjected to bonding operation due to the fact that the crystallization temperature is not very different from this heat treatment temperature, resulting in unwanted loss of the magnetic properties. Also, an organic material (for example, quinondiazide photosensitive resin (AZ-2350)), which is often used as an interlayer insulator for thin film magnetic heads, starts volume shrinkage at about 300.degree. C. and thus cannot withstand the glass bonding operation.
An alternative method is also known in which an organic adhesive (for example, an epoxy resin known under the tradename "Araldite") is used to bond the protective plate. This method makes it possible to keep the curing temperature of this organic adhesive at less than 200.degree. C., avoiding the high bonding temperature. The adhesive bonding has a drawback that, though milder than the glass bonding, is less reliable.